In order to improve the encoding efficiency, ITU-T H.264, which is a standard video encoding method (see, for example, Non-Patent Document 1), employs a prediction encoding method in which a predicted signal is generated using peripheral pixels in intra encoding.
In the intra prediction encoding of H.264, the prediction mode or the prediction block size can be adaptively switched. In accordance with such a prediction encoding method, the encoding efficiency of intra encoding has been remarkably improved in comparison with the other known video encoding methods.
FIGS. 6A to 8B show the prediction mode and the prediction block size, employed in H.264.
FIGS. 6A and 6B relate to an intra prediction encoding of a target macroblock (16×16 pixels) using a prediction block size of 4×4 pixels, where FIG. 6A shows the positional relationship between the target blocks (for encoding) and already-encoded blocks, and FIG. 6B shows the prediction direction.
Similarly, FIGS. 7A and 7B relate to an intra prediction encoding of a target macroblock using a prediction block size of 8×8 pixels, where FIG. 7A shows the positional relationship between the target blocks (for encoding) and already-encoded blocks, and FIG. 7B shows the prediction direction.
Similarly, FIGS. 8A and 8B relate to an intra prediction encoding of a target macroblock using a prediction block size of 16×16 pixels (i.e., the same size as the macroblock), where FIG. 8A shows the positional relationship between the target block (for encoding) and already-encoded signals (or pixels), and FIG. 8B shows predictions by means of various directions and methods.
As described above, in H.264, any prediction block size and any prediction direction can be selected in accordance with the encoding efficiency.
However, although an increase in the number of selectable prediction modes or prediction block sizes improves the encoding efficiency, it increases the cost of computation performed for selecting the prediction mode or prediction block size. Therefore, a method for reducing the computation cost for intra prediction encoding has been proposed.
In the invention disclosed in Non-Patent Document 1, in accordance with the flowchart in FIG. 9, the edge direction is first determined; limited prediction modes are provided; a prediction mode is selected; the prediction block size is determined; and then intra prediction encoding is performed.
In the above-described invention disclosed in Non-Patent Document 1, the edge direction is measured in advance so as to exclude prediction modes having a low probability of being selected, thereby reducing the computation cost.
In the invention disclosed in Non-Patent Document 2, in accordance with the flowchart in FIG. 10, the prediction mode for the 4×4 block size is first selected; a bias for the prediction mode is measured; based on the measurement results, selection of the prediction mode for the 8×8 block size may be skipped, or the prediction mode for the 8×8 block size (if there is no skipping for this size) or 16×16 block size may be performed; the prediction block size is determined; and then intra prediction encoding is performed.
In the above-described invention disclosed in Non-Patent Document 2, the prediction mode for the 4×4 block size (i.e., minimum block size) is first selected, and based on the appearance frequency of the prediction mode for the 4×4 block size, limitation on the 8×8 or 16×16 block size and selection of the prediction block size are controlled.
That is, in the invention disclosed in Non-Patent Document 2, the prediction mode for the 8×8 block size may be limited or the prediction for the 8×8 block size may be skipped in accordance with a bias for the minimum block size, so as to reduce the computation cost.
Additionally, in the invention disclosed in Patent Document 1, frequency data for the prediction mode of peripheral blocks is weighted, and the prediction mode is limited based on the weighted frequency data.
In the invention disclosed in Patent Document 1, the prediction mode and a weighting factor for a reference block are first obtained, and then the priority order of the prediction mode is obtained. Based on the obtained prediction mode and the weighting factor, a weighted histogram for the prediction mode is produced. Then, based on the weighted histogram, a candidate for the prediction mode is selected. In this process, if there is no prediction mode which satisfies a predetermined number used for determining the candidate (as the prediction mode), a prediction mode having a relatively higher priority order is selected, so as to perform intra prediction encoding.
As described above, in the invention disclosed in Patent Document 1, the prediction mode to be used is limited based on statistical data for the prediction mode of peripheral blocks, thereby reducing the number of execution times of a cost computation which is required for selecting the prediction mode.    Non-Patent Document 1: Feng Pan et al., “Fast Mode Decision Algorithm for Intraprediction in H.264/AVC Video Coding,” IEEE Trans. Circuits Syst. Video Technol., vol. 15, No. 7, pp. 813-822, July 2005.    Non-Patent Document 2: Yuichi Tsunematsu et al., “Fast Intra Mode Decision for H.264/AVC FRExt Using Prediction Result Bias”, Proceedings of the 2005 IEICE general conference, D-11-5, p. 55, March 2005.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2005-348280.